Various types of spark plugs which have multi-element insulators are known. German Patent DE-PS No. 267 606 describes a spark plug which has a metal housing in which a multi-part insulator is positioned. The subdivision of the insulator elements is transverse to the longitudinal extent of the spark plug, so that the insulator element closest to the ignition end can be removed and replaced in case of damage thereto.
German Patent Disclosure Document DE-OS No. 30 01 711 to which U.S. Ser. No. 214,481, now U.S. Pat. No. 4,393,687 corresponds, describes a spark plug having a pre-ignition chamber in which a center electrode is used formed as a conductive track on the inside of an insulator which covers the inside of the pre-ignition chamber. The conductive track has interruptions to form a pre-sparking gap. The center electrode is connected at the terminal end to a metallic rod or post, secured in the spark plug insulator.
German Patent Disclosure Document DE-OS No. 31 09 896 describes a different type of spark plug having a pre-ignition chamber in which a layer-like center electrode is applied on the end portion of the insulator at the ignition side, the layer electrode being connected to a metal wire which leads, as well known, through an axial inner bore of the insulator.
Japanese Patent Disclosure Document No. 153 046/77 describes a spark plug in which the electrical connection between a terminal or connecting bolt or post and the center electrode includes an electrical resistance layer which is applied on the surface of the insulator within the inner axial opening thereof.
Various types of spark plugs have been proposed which include electrical heater elements at the ignition end portion thereof. For example, and as shown in German Patent Disclosure Document DE-OS No. 23 10 586, such a heater element may be secured directly within the center electrode. U.S. Pat. No. 2,046,650 describes a heater in the form of a heater spiral located between the insulator and the metallic housing. U.S. Pat. No. 2,130,208 shows a heater which is secured, in ring form, within the metal housing and spaced from the metallic center electrode. U.S. Pat. No. 1,784,541 shows a spark plug in which a heater is positioned within the metal housing ahead of the spark gap. British Pat. No. 314,307 shows a spark plug in which a heater is located within the metal housing at the ignition end portion thereof.
All the various constructions of spark plugs with heater exemplified by the referenced patents can be constructed only with substantial expense, or have a heat capacity which is high; or are subject to misfires and/or malfunction.
The spark plug, which is screwed through the cylinder head of an IC engine provides a convenient unit to hold not only the actual spark gap, but additional structures and arrangements which are desirable or useful to have to sense the combustion conditions occurring within the combustion chamber. German Patent Disclosure Document DE-OS No. 30 28 188 to which U.S. Ser. No. 346,034, filed Jan. 28, 1982 DOBLER et al., corresponds describes, for example, the combination of a pressure sensor with a spark plug, located at the ignition end portion of the spark plug. To provide a pressure output, a pressure sensitive wire is used. Another type of spark plug which includes a pressure sensor is described in German Patent DE-PS No. 1 015 623, which utilizes a piezo-electrical element as the pressure sensor.
Other types of sensors may be combined with spark plugs, thereby avoiding the necessity of forming the cylinder chamber of the IC engine with which they are to be used with yet another opening, which has to be threaded and sealed by the sensor. For example, German Patent Disclosure Document DE-OS No. 29 30 013 describes a spark plug in combination with an ion current sensor which measures the electrical conductivity of the gases between adjacent electrodes.
Oxygen sensors can be combined with spark plugs, see for example German Patent Disclosure Document DE-OS No. 30 28 359, which describes a spark plug having an oxygen sensor positioned adjacent the ignition end portion thereof. To measure the partial oxygen pressure, a rotation-symmetrical sensor is embedded at the tip portion of the insulator, and connected to the connection portion of the insulator over platinum-conductive tracks, which are introduced into suitable openings in the form of a platinum suspension, dripped into the openings. The sensor element itself is an oxygen ion conductive solid electrolyte body, such as for example zirconium dioxide, supplied with suitable electrodes in layered or sandwich construction.
Sensing of temperature in the combustion chamber has also been proposed--see, for example, German Utility Patent DE-GM No. 70 01 121, which describes a thermal element integrated with a spark plug. This publication includes references to other features which can be used in combination with spark plugs.
It has been found that the constructions described in all the aforementioned publications result in sensor-spark plug combinations which are expensive to make, and require, for suitable operation and efficient use, more space than usually available at the ignition end of a spark plug. It is hardly ever possible to combine more than one additional element besides the center electrode and the conduction lines therefor with the terminal portion of the spark plug; thus, combination of a heater and a sensor is not practical.